The present invention is related to integrated circuits in general and, in particular, to a monolithic integrated circuit device or switch comprising means forming a capacitive diode and a small field-effect transistor connected to the gate of a switching junction field-effect transistor (junction FET or JFET).
The N-channel junction FET having, for example, the UN5001B geometry, has long been a very popular device for use in solid state (analog gate) switching. It has a low drain-source switch resistance, R.sub.DS(ON), as of the order of 40 ohms, coupled with a low pinch-off voltage V.sub.p, as of the order of 2.5 volts. The low V.sub.p allows a user to switch signals of .+-.10 volts with a .+-.15 volt power supply and still have at least a volt of margin so that with V.sub.p .ltoreq. 4 volts, up to .+-.11 volts can be switched with .+-.15 volts. The limiting parameter on the signals that can be effectively handled is V.sub.p. This is due to the fact that gate voltage must exceed (in a negative direction) source or drain voltage by V.sub.p, or the switch will not turn off. Thus, if -10 volts is the input signal, and V.sub.p = 4 volts, gate voltage must be at least -14 volts.
While the UN5001B or equivalent has characteristics that are favorable for use in switching, the fact that it is an N-channel depletion mode device means that a driver circuit must be used to translate positive logic (i.e., TTL, DTL, RTL) into voltages required to drive the FET. This normally means a +3 volts to +15 volts translation and a 0 volt to -15 volts translation.
Because of the voltage translation required, some sort of isolator is also required between the translator and the output of the FET so that the source to gate junction (or drain to gate junction) of the FET will not be forward biased during switching. This isolator is usually provided as a separate diode in parallel with a separate capacitor or a diode in conjunction with a gate to source referral resistor. The diode may be of the simple type with a gradual curve of decreasing junction capacitance or of the "pinch"-type where the capacitance decreases gradually until the pinch voltage is reached at which point the capacitance decreases very rapidly with further increase of voltage.
In addition to being inherently more costly because of the time and expense of manufacturing separate parts and then having to assemble the parts, the prior known hybrid circuit devices are also subject to the possibility of reduced performance due to current leakage associated generally with external connections.